Field
Embodiments described herein relate generally to data storage units, systems, and methods for accurate spiral gate positioning in the presence of large non-repeatable runout.
Description of the Related Art
In a typical hard disk drive (HDD), servo sectors on the disk are used to provide position information about the location of a magnetic head over a disk surface. A common approach for writing such servo information on each disk surface in an HDD is referred to as spiral-based self-servo writing, or spiral-based SSW. According to this approach, spiral-shaped servo information (or “servo spirals”) is written on at least one disk surface prior to the SSW process. During the SSW process, a spiral-based servo system is employed to write the servo sectors, in which each magnetic head of the HDD is positioned relative to a disk surface based on the servo spirals. In this way, the final servo information on each disk surface can be written by the magnetic heads.
In a spiral based servo system, the read channel and drive firmware of the HDD determine the head position based on the timing and amplitude characteristics of the signal generated when the head crosses a spiral. Two possible approaches for detecting such spiral crossings are continuous spiral demodulation, also referred to as spiral search mode, and windowed spiral demodulation.
In continuous spiral demodulation, the read channel continuously monitors the signal from the head and uses a suitable algorithm to detect when a spiral crossing occurs. A serious disadvantage of this approach is that stray signals left on the media may be misinterpreted as spiral crossings, causing erratic servo behavior and even failure of the HDD. In windowed spiral demodulation, the read channel monitors spiral crossings only during predefined and relatively short time intervals, typically referred to as spiral gates, thereby greatly reducing the potential for stray signals from affecting servo behavior. Windowed spiral demodulation mode requires approximate knowledge of when each spiral crossing will occur, so that the spiral demodulation window for that spiral can be timed to coincide with when the spiral is being crossed by the head. Typically, spiral locations on the media can be characterized accurately by suitable firmware algorithms, and spiral demodulation windows can be reliably activated near the appropriate spiral crossing. However, when the head experiences high non-repeatable runout (NRRO), significant unpredictable variation of spiral crossing times generally results, causing missed spiral detection. High NRRO is common for a head servoing on widely spaced spirals, such as the so-called “bootstrap spirals” that are written on a disk as part of a blank disk SSW process.
In light of the above, neither windowed spiral demodulation nor continuous spiral demodulation can provide robust spiral detection in an SSW process that uses widely spaced spirals. Accordingly, there is a need in the art for a method of positioning a magnetic head during an SSW process in the presence of large NRRO of the head position.